Molybdenum is an essential trace element and is also toxic at high intake. The proposed research is designed to extend our present knowledge of the physiology of molybdenum at "normal" and "near normal" levels in mammals, specifically in rats. We will study; 1) the relative bioavailability of molybdenum in food and in water; 2) the sites of gastro-intestinal absorption, and 3) the kinetics of absorption and elimination from the body and kinetics of uptake, retention, and release from selected tissues. The fact that molybdenum appears in the diet in three or more forms (in water as the molybdate ion, in food as the molybdate-cysteine complex in enzymes, and in unknown storage and transport forms) suggests that there might be important differences in the modes and routes of absorption and elimination. Extensive data gathered by my laboratory and other laboratories on (MoO4)2 minus, using Mo-99, show rapid and essentially complete absorption in the stomach and small intestine. Our working hypotheses are: 1) that absorption of molybdenum from food is in the small intesting only, 2) that absorption will be slower from food than from water, and 3) that less will be absorbed from plant material. We will work with the molydbate ion Mo6 ion) and the molybdate-cysteine complex (Mo5 ion) both as Mo-99, to determine the kinetics of movement. Plants will be allowed to take up Mo-99 and then the leaves and stems will be fed to rats to determine if the molybdenum in plants behaves as the molybdate-cysteine complex. The proposed approaches have not yet been used in research on molybdenum nor on many other trace metals. They should give much new information on the behavior of this essential trace element in the body of mammals.